Secret Intelligence Service
The Secret Intelligence Service (SIS) is responsible for supplying the British Government
Government of the United Kingdom
Her Majesty's Government is the central government of the United Kingdom of Great Britain and Northern Ireland. The Government is led by the Prime Minister, who selects all the remaining Ministers...

 with foreign intelligence
Intelligence agency
An intelligence agency is a governmental agency that is devoted to information gathering for purposes of national security and defence. Means of information gathering may include espionage, communication interception, cryptanalysis, cooperation with other institutions, and evaluation of public...

. Alongside the internal Security Service
The Security Service, commonly known as MI5 , is the United Kingdom's internal counter-intelligence and security agency and is part of its core intelligence machinery alongside the Secret Intelligence Service focused on foreign threats, Government Communications Headquarters and the Defence...

 (MI5), the Government Communications Headquarters
Government Communications Headquarters
The Government Communications Headquarters is a British intelligence agency responsible for providing signals intelligence and information assurance to the UK government and armed forces...

 (GCHQ) and the Defence Intelligence (DI), it operates under the formal direction of the Joint Intelligence Committee (JIC).

It is frequently referred to in the mass media and popular parlance by the name MI6, a name used as a flag of convenience during the Second World War when it was known by many names.